Methods of fabricating large-area, semiconducting nanoperforated graphene materials

ABSTRACT

Methods for forming a nanoperforated graphene material are provided. The methods comprise forming an etch mask defining a periodic array of holes over a graphene material and patterning the periodic array of holes into the graphene material. The etch mask comprises a pattern-defining block copolymer layer, and can optionally also comprise a wetting layer and a neutral layer. The nanoperforated graphene material can consist of a single sheet of graphene or a plurality of graphene sheets.

CROSS-REFERENCE TO RELATED APPLICATION

The present application is a continuation of U.S. patent applicationSer. No. 13/013,531 that was filed Jan. 25, 2011, which claims priorityfrom U.S. provisional Patent Application No. 61/298,302, that was filedJan. 26, 2010, the entire contents of which are incorporated herein byreference.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

This invention was made with government support under 0520527 and0425880 awarded by the National Science Foundation. The government hascertain rights in the invention.

BACKGROUND

Scientific and technological interest in graphene has rapidly grownrecently because of the extraordinary electronic properties of thetwo-dimensional material. (See, Geim, A. K. Science 2009, 324,1530-1534.) The mean free path for electron-phonon scattering ingraphene is astonishingly long (>2 mm), and as a result, the roomtemperature electronic mobility in graphene could potentially exceed200,000 cm² V⁻¹ s⁻¹ if scattering from disorder in the extrinsicenvironment were to be minimized. (See, Morozov, S. V.; Novoselov, K.S.; Katsnelson, M. I.; Schedin, F.; Elias, D. C.; Jaszczak, J. A.; Geim,A. K. Phys. Rev. Lett. 2008, 100, 016602; Chen, J. H.; Jang, C.; Xiao,S. D.; Ishigami, M.; Fuhrer, M. S. Nat. Nanotechnol. 2008, 3, 206-209.)Next-generation, ultrahigh performance electronics and transistor logiccircuits are envisioned that exploit these exceptional properties. (See,Wang, X. R.; Ouyang, Y. J.; Li, X. L.; Wang, H. L.; Guo, J.; Dai, H. J.Phys. Rev. Lett. 2008, 100, 206803.) Other potential electronicapplications of graphene as transparent conductors, sensors, and inflexible electronics have also been demonstrated and proposed. (See,Kim, K. S.; Zhao, Y.; Jang, H.; Lee, S. Y.; Kim, J. M.; Ahn, J. H.; Kim,P.; Choi, J. Y.; Hong, B. H. Nature 2009, 457, 706-710; Schedin, F.;Geim, A. K.; Morozov, S. V.; Hill, E. W.; Blake, P.; Katsnelson, M. I.;Novoselov, K. S. Nature Mat. 2007, 6, 652-655.)

Unfortunately, however, despite its excellent charge transportcharacteristics, the applicability of graphene in many electronicapplications is currently limited because graphene does not have atechnologically significant band gap >>kT. (See, Castro Neto, A. H.;Guinea, F.; Peres, N. M. R.; Novoselov, K. S.; Geim, A. K. Rev. Mod.Phys. 2009, 81, 109-162.) The insufficient band gap limits how stronglythe conductance of graphene-based devices can be modulated by extrinsicor field-effect doping—which is a critically important behavior forsemiconductor applications.

To address this problem, it has been shown that quantum confinementeffects can be used to open up a band gap in graphene. For example, ithas been demonstrated that the band gap of graphene nanoribbons, E_(g),patterned using electron-beam lithography, roughly varies inversely withthe width of the nanoribbons, w, according to E_(g)˜0.2-1.5 eV-nm/w.(See, Castro Neto, A. H.; Guinea, F.; Peres, N. M. R.; Novoselov, K. S.;Geim, A. K. Rev. Mod. Phys. 2009, 81, 109-162; Stampfer, C.; Gutttinger,J.; Hellmueller, S.; Molitor, F.; Ensslin, K.; Ihn, T. Phys. Rev. Lett.2009, 102, 056403; Yang, L.; Park, C. H.; Son, Y. W.; Cohen, M. L.;Louie, S. G. Phys. Rev. Lett. 2007, 99, 186801.) Other forms ofnanostructured graphene showing semiconducting behavior have also beenfabricated using electron-beam lithography, including graphene quantumdots. (See, Ponomarenko, L. A.; Schedin, F.; Katsnelson, M. I.; Yang,R.; Hill, E. W.; Novoselov, K. S.; Geim, A. K. Science 2008, 320,356-358.) and inverse dot (See, Eroms, J.; Weiss, D. arXiv:0901.0840;Shen, T.; Wu, Y. Q.; Capano, M. A.; Rokhinson, L. P.; Engel, L. W.; Ye,P. D. Appl. Phys. Lett. 2008, 93, 122102.)

The successes of electron-beam lithography in fabricating graphenenanostructures that exhibit semiconducting behavior have highlighted twofuture challenges. First, in order to open a band gap >>kT innanostructured graphene, it must be nanopatterned to critical dimensions<20 nm. However, 20 nm is on the threshold of what can easily beachieving using conventional electon beam lithography due to knownelectron scattering effects in common electron-beam resists. Morerecently with an experimental electron beam resist system features downto 10 nm have been demonstrated. (See, Miyazaki, T.; Hayashi, K.;Kobayashi, K.; Kuba, Y.; Ohyi, H.; Obara, T.; Mizuta, O.; Murayama, N.;Tanaka, N.; Kawamura, Y.; Uemoto, H. J. Vac. Sci. Technol. B 2008, 26,2611.) However the second challenge is that, electron-beam lithographyis a serial technique, which limits its throughput and applicability tothe large-area patterning of graphene. Thus, in order to morepractically realize nanostructured graphene-based materials, newpatterning techniques that can be extended to large-areas with sub-20 nmresolution are needed.

SUMMARY

One aspect of the present invention provides methods for forming ananoperforated graphene material. The methods comprise forming an etchmask defining an array of holes over a graphene material and patterningthe array of holes into the graphene material, wherein the etch maskcomprises a wetting layer in contact with the graphene material, aneutral layer comprising a copolymer disposed over the wetting layer,and a pattern-defining copolymer layer disposed over the neutral layer.The nanoperforated graphene material can consist of a single sheet ofgraphene or a plurality of graphene sheets. For example the graphenematerial can be highly oriented pyrolytic graphite (HOPG).

In some embodiments of these methods, the wetting layer comprises aself-assembled monolayer (SAM). The SAM is desirably comprised ofmolecules having a first end that is sufficiently polar to undergo anattractive interaction with the copolymer of the neutral layer and asecond end comprising a plurality of conjugated rings that undergo a π-πstacking interaction with the graphene material. For example, the firstend of the molecules can comprise an acid group and second end of themolecules can comprise a plurality of fused aromatic rings. An exampleof a suitable SAM molecule is pyrene butyric acid. In other embodiments,the wetting layer comprises silicon oxide.

Another aspect of the invention provides methods for forming ananoperforated graphene material, the methods comprising coating apattern-forming block copolymer onto a graphene material and exposingthe deposited pattern-forming block copolymer to a vapor atmospheresaturated with an annealing solvent for a time sufficient to allow thepattern-forming block copolymer to self assemble into oriented domains.At least some of the oriented domains can then be selectively removed toform an etch mask defining an array of holes, which can be patternedinto the underlying graphene material. In some embodiments of thesemethods, the pattern-defining block copolymer comprises acylinder-forming diblock copolymer of styrene and methyl methacrylateand the annealing solvent comprises carbon disulfide.

Yet another aspect of the invention provides methods for forming ananoperforated graphene material, the methods comprising forming an etchmask defining an array of holes on a substrate, wherein the etch maskcomprises a wetting layer in contact with the substrate, a neutral layercomprising a copolymer disposed over the wetting layer, and apattern-defining block copolymer layer comprising a plurality of ordereddomains disposed over the neutral layer. The wetting layer and theneutral layer are then selectively removed from the etch mask and thepattern-defining block copolymer layer is separated from the substrate.The resulting released pattern-defining block copolymer layer is thentransferred onto a graphene material and the array of holes is patternedinto that material.

Still another aspect of the invention provides methods for forming ananoperforated graphene material which include the steps of forming anetch mask over a graphene material, wherein the etch mask comprises aprotective layer comprising an uncrosslinked polymer in contact with thegraphene material, an etch stop layer disposed over the protectivelayer, and a pattern-defining block copolymer layer comprising aplurality of ordered domains disposed over the etch stop layer;selectively etching away at least some of the domains to form an arrayof holes in the pattern-forming block copolymer layer, wherein the etchstop layer is comprised of a material that is resistant to the etchantused to etch away the domains; patterning the array of holes through theetch stop layer, the protective layer and the graphene material in oneor more subsequent etching steps; and removing the remaining etch stoplayer and protective material from the graphene material. In someembodiments of these methods, the uncrosslinked polymer comprisesuncrosslinked polystyrene, the etch stop layer comprises silicon oxideand the pattern-forming block copolymer comprises a cylinder-formingdiblock copolymer of styrene and methyl methacrylate. In some suchembodiments, the etchants used to etch away at least some of the domainsin the pattern-forming block copolymer comprise ultraviolet radiationand an oxygen plasma, the etchant used to pattern the array of holesthrough the etch stop layer comprises a fluorine-containing gas, and theetchant used to pattern the array of holes through the protective layerand the graphene material comprises an oxygen plasma. Using the presentmethods, the features of the periodic array of holes (including the holediameters, hole spacing and/or constrictions between the holes) in thenanoperforated graphene material can be fabricated with dimensions ofless than 20 nm, or even less than 10 nm. In some embodiments, thefeatures of the periodic array of holes in the nanoperforated graphenematerials are dimensioned such that they provide a semiconductingmaterial having an electronic band gap of at least about 100 meV.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1. (Left Panel) Schematic depicting the fabrication ofnanoperforated HOPG using block copolymer lithography and thecorresponding top-down scanning electron microscope (SEM) images of (a)vertically oriented polymethylmethacrylate (PMMA) cylinders in a blockcopolymer thin film obtained by thermal annealing, (b) residualpolystyrene (PS) honeycomb template obtained after selective PMMAremoval with UV irradiation, (c) etched structures after O₂ followed byCHF₃+O₂ plasma reactive ion etch (RIE) resulting in the etching of therandom copolymer mat and the oxide buffer layer respectively, and (d)nanoperforated HOPG resulting from the final O₂ plasma RIE and theremoval of the oxide buffer by HF solution.

FIG. 2. Physical and chemical characterization of nanopatterned HOPG.(a) The histogram and distribution of critical feature dimensions. Thehole diameter, the period of the repeating pattern, and the constrictionwidth are labeled d, D, and w, respectively. Inset depicts thenanoperforated graphene structure. (b) x-ray photoelectron spectroscopy(XPS) of patterned oxide layer after final O₂ plasma RIE step (i), afterCHF₃+O₂ RIE step and oxide layer removal with HF (ii), after final O₂plasma RIE step and oxide layer removal with HF (iii). (c) Ramanscattering of unpatterned HOPG (bottom curve); patterned graphene afterCHF₃+O₂ RIE step, skipping the final O₂ plasma RIE step, and oxide layerremoval with HF (middle curve); and patterned graphene following thefull procedure depicted in FIG. 1 including the final O₂ plasma RIE stepand oxide layer removal with HF (top curve).

FIG. 3. Electronic characterization. (a) Schematic of the nanoperforatedgraphene field effect transistor device. (b) Top-down SEM image of afabricated device. Scale bar=100 nm. (c) Measured graphene conductanceversus gate bias for a device before nanopatterning measured at roomtemperature (dashed curve); device after nanopatterning but before finalO₂ RIE step measured at room temperature (dotted curve); device afternanopatterning and after final O₂ RIE step measured at room temperature(dash/dotted curve) and at T=105 K (solid curve). (d) Minimumconductance versus 1/T fit to a straight line, indicating that aneffective gap of 102 meV has opened as a result of nanopatterning.Insert shows the source-drain current as a function of source drain biasat V_(G)−V_(Dirac)=−30, −20, −10, and 0 V, from bottom to top,respectively, at 105 K.

FIG. 4. Top-down SEM image of dewetted P(S-b-MMA) thin film. The randomcopolymer was formed by direct deposition of P(S-r-MMA-r-GMA) solutionon to the surface of HOPG.

FIG. 5. Large area SEM images of (a) perpendicular PMMA cylinders on agraphene material substrate. (b) Honey comb structures after final shortO₂ plasma RIE step.

FIG. 6. Schematic illustration of the direct transfer of apattern-forming block copolymer layer onto an underlying graphenesubstrate.

FIG. 7. Schematic illustration of the tri-layered approach to formingnanopatterned graphene, as described in Example 5.

FIG. 8 SEM images of etch masks comprising pyrene butyric acid SAMwetting layers on graphene (top panels) and nanoperforated graphenelayers patterned using the etch masks (bottom panels), in accordancewith Example 3.

DETAILED DESCRIPTION

One aspect of the invention provides methods for nanopatterning graphenematerials using block copolymer (BCP) lithography. BCP lithography canbe facilely scaled to batch-process multiple, large-area substrates inparallel while simultaneously achieving the high-fidelity patterning ofexceptionally small features, including features with dimensions of lessthan 10 nm. The resulting material is a novel form of nanopatternedgraphene referred to as nanoperforated graphene. In one basic embodimentof the methods, nanoperforated graphene materials are fabricated using adiblock copolymer-based etch mask to transfer the pattern of a periodicarray of holes into an underlying grapheme substrate via selective andcontrolled etching. Because the present methods can be implemented andscaled to large areas and can control the electronic properties ofgraphene, they can also enable practical large-area, commercializableapplications of graphene in electronics, thin film electronics, flexibleelectronics, optoelectronics, nanofiltration, photonics and sensing. Thenanopatterned graphene materials may be particularly well-suited fornanofiltration applications because, in some embodiments, they are onlyan atomic layer in thickness, but still strong enough to withstandconsiderable pressure. As such, they have minimal impedance to fluidflow—which is highly desired for filtration. In addition, as describedbelow, the hole sizes are tunable. Photonics are another attractiveapplication for the present materials because their band gaps aretunable and, therefore, can be tailored to absorb/emit/detect light atvarious energies depending on the pore size and periodicity and thedetails of their structure.

Materials that can be patterned with the present methods include singlegraphene sheets and materials that include a plurality of graphenesheets. For example, the present methods can be used to pattern HOPGwhich has interlayer bonds, or other forms of multilayered graphite orgraphene.

The dimensions of the pattern transferred into the graphene material canbe characterized by the diameters of the holes, the hole spacing (asmeasured between hole centers) and/or the widths of the constrictionsbetween the holes. In some embodiments, the present methods can be usedto form a pattern of holes in which one, two, or all three of thesedimensions are less than about 40 nm. This includes embodiments in whichone, two, or all three of these dimensions are less than about 30 nm,further includes embodiments in which one, two, or all three of thesedimensions are less than about 20 nm, and still further includesembodiments in which one, two, or all three of these dimensions are lessthan about 10 nm. Using the present methods, the dimensions of thepattern in the nanoperforated graphene material can be tailored toprovide a semiconducting material. For example, nanoperforated graphenematerials having an electronic band gap of at least about 100 meV can befabricated. These dimensions can be tailored by altering the molecularweight of the BCP from which the etch masks are made, as illustrated inExample 2, below. Typically, reducing the molecular weight of the BCPleads to reduced etching dimensions.

The BCP lithography-based methods can produce nanoperforated graphenematerials with large nanoperforated areas. For example, in someembodiments the materials have a nanoperforated area of at least 1 mm2.

In one embodiment, the etch mask used in the methods can be fabricatedfrom a multilayered structure that includes a wetting layer, a neutrallayer disposed over the wetting layer and a pattern-defining layerdisposed over the neutral layer.

The wetting layer is a thin film of material that serves to improve thewetting of the neutral layer on the graphene material, therebypreventing the dewetting of the overlying pattern-defining layer thatwould otherwise occur. The use of this layer is advantageous due to thepoor wettability of graphene by solvents and polymers used to form theother layers of the etch mask. In some embodiments, silicon oxide isused as the wetting layer. This wetting layer can be deposited directlyonto the graphene material and is typically quite thin. For example, insome embodiments the wetting layer has a thickness about 5 to 20 nm,although thicknesses outside of this range may also be employed.

Wetting layers made from self-assembled monolayers (SAMs) can also beused. The molecules making a SAM wetting layer are organic moleculescharacterized by a first end that is sufficiently polar to undergo anattractive interaction with an overlying neutral layer. In someembodiments, the first end of the molecules comprises an acid group,such as a carboxyl group. The second end of the molecules comprising theSAM are characterized in that they are capable of undergoing anattractive interaction, such as a π-π stacking interaction, with theconjugated rings of the underlying graphene. This interaction should besufficiently strong to prevent the SAM from being washed away bysolvents used in processing the etch masks. In some embodiments thesecond end of the molecules comprises a conjugated ring system,desirably composed of three or more aromatic rings, such as fusedsix-member aromatic rings (e.g., benzene rings). One example of asuitable second end group is pyrene and one example of a suitablemolecule for a SAM wetting layer is pyrene butyric acid (PBA). Anadvantage of replacing an oxide layer with a SAM as the wetting layer isthat it can eliminate the need for more than one etchant during thefabrication of the etch mask. For example, for masks in which each ofthe layers (i.e., the wetting layer, the neutral layer and thepattern-defining layer) is comprised of organic materials, the masks canbe etched through with an O₂ plasma, without the need to resort to morecaustic etchants, such as fluorine-containing gases. Another advantageof SAM-based wetting layers is that they provide greater tunability ofthe wettability.

The neutral layer is formed from a material that induces patternformation in the overlying block copolymer pattern-defining layer. Forexample, in one embodiment the neutral layer induces the formation ofcylindrical domains within the overlying block copolymer layer, asdescribed in greater detail in Example 1, below. The neutral layermaterial can be a copolymer polymerized from vinyl monomers and acrylatemonomers. An example of one such copolymer is a random copolymer ofmethyl methacrylate (MMA), styrene (S) and glycidyl methacrylate (GMA),P(S-r-MMA-r-GMA). The ratios of the monomers that make up the neutrallayer polymer can vary, depending on the desired characteristics of theetch mask pattern. By way of illustration only, in some embodiments, theweight percent of styrene in a P(S-r-MMA-r-GMA) neutral layer rangesfrom about 70 to 85 percent, the weight percent of polymethylmethacrylate in a P(S-r-MMA-r-GMA) neutral layer ranges from about 10 to30 percent, and the weight percent of glycidyl methacrylate in aP(S-r-MMA-r-GMA) neutral layer ranges from about 2 to 5 percent.

The block copolymer pattern-defining layer forms a periodic pattern dueto the phase segregation of the copolymer into a regular pattern ofdomains. Block copolymers from which the pattern-defining layer can beformed include block copolymers of vinyl monomers and acrylate monomers.For example, the pattern-defining layer can be formed from a blockcopolymer of styrene and methylmethacrylate, P(S-b-MMA) which formsvertically oriented, hexagonally-packed cylindrical domains. Themolecular weight of the components that make up the pattern-forminglayer polymer can vary, depending on the desired characteristics of theetch mask pattern. By way of illustration only, in some embodiments, thenumber average molecular weight of the polystyrene in a P(S-b-MMA)pattern-forming layer ranges from about 20,000 to 50,000, while thenumber average molecular weight of the polymethyl methacrylate rangesfrom about 8,000 to 25,000.

An etch mask can be formed from the multilayered structure byselectively removing (e.g., etching) domains in the block copolymerlayer to provide a pattern-defining layer that defines an array of holescorresponding to the domain pattern in the block copolymer layer andetching the array into the remaining layers of the etch mask. Theresulting pattern can then be transferred into the underlying graphenematerial, as illustrated in the examples, below.

In some embodiments of the present methods, both the wetting layer andthe neutral layer can be eliminated from the etch mask structure byusing a solvent anneal to deposit the pattern-forming layer directlyonto the graphene substrate. During the solvent anneal, the BCP filmundergoes swelling as it is exposed to a saturated solvent vaporatmosphere, typically at room temperature (28° C.), for a timesufficient to allow the BCP to self-assemble into oriented domains. Insome embodiments the solvent anneal is conducted for at least 8 hours.This includes embodiments in which the solvent annealing is conductedfor at least 10 hours. Certain aspects of these embodiments can beattributed to the inventors' discovery that solvents, such as carbondisulfide (CS₂) can be used as annealing solvents to create acylindrical domain polymer morphology in a P(S-b-MMA) BCP on graphene.Other annealing solvents include tetrahydrofuran (THF), acetone,chloroform, tolune, and mixtures thereof. The solvent annealing methodis illustrated in greater detail in Example 3, below. Parameters thateffect the development of morphology by solvent annealing arepolymer-solvent interaction parameters (χ_(P-S)), the film thickness andthe exposure time to solvent vapors. By way of illustration, for theCS₂, these parameters may be as follows: (χ_(PS-S)=0.43,χ_(PMMA-S)=1.2), PS-selective solvent, swells PS block preferentially.For other solvents such as, THF (χ_(PS-S)=0.34, χ_(PMMA-S)=0.88),acetone (χ_(PS-S)=1.1, χ_(PMMA-S)=0.29), chloroform (χ_(PS-S)=0.39,χ_(PMMA-S)=0.45), and toluene (χ_(PS-S)=0.34, χ_(PMMA-S)=0.45). Mixedsolvent vapors can also be used to change the morphology of blockcopolymer thin films on graphene. A film thickness less than (½ L₀) istypically preferred to form morphology without defects. Furthermore,exposure time can be optimized to avoid dewetting of the film from thesubstrate, which can occur if the exposure time is too long. (See, Xuan,Yu.; Peng, J.; Cui, L.; Wang, H.; Li, B.; Han, Y. Macromolecules 2004,37, 7301-7307; Peng, J.; Kim, D. H.; Knoll, W.; Xuan, Y.; Li, B.; Han,Y. J. Chem. Phys. 2006, 125, 064702.)

In other embodiments of the present methods, a wetting layer and aneutral layer are used to create a pattern-forming layer having thedesired morphology, but the wetting layer and neutral layer are removedprior to the selective removal of domains to form the final etch maskpattern. This approach is illustrated schematically in FIG. 6. As shownin the figure, a wetting layer 602 and a neutral layer 604 are depositedonto an underlying graphene substrate 606. A BCP pattern-forming layer608 is then deposited over the neutral layer, where it assembles into aseries of domains forming a pattern in the layer. The structure can thenbe immersed in a liquid or vapor phase solution 610 capable of removing(e.g., etching away) the wetting layer and the neutral layer. Theremaining, released pattern-forming layer can then be transferreddirectly back on to the graphene substrate 606.

Various devices, such as field effect transistors (FETs), can befabricated from the nanoperforated graphene made using the present etchmask structures and methods. In some instances, components, such asmetal contacts, can be deposited onto the graphene substrate prior topatterning. However, in other instances it is desirable to deposit suchcomponents after the patterning of the graphene is complete. In suchinstances, a variation of the methods, referred to as the “tri-layerapproach” can be used. This approach takes advantage of thepattern-forming layer transfer technique, described above, and inserts a“protective layer” over the graphene substrate to protect the graphenefrom etchants used during the processing of the etch mask and tofacilitate the complete the removal of the etch mask from the grapheneafter patterning is complete. The tri-layer process is described indetail in Example 5, below. FIG. 7 provides a schematic illustration ofthe tri-layer approach. In a first step, graphene 702 is deposited(e.g., spin-coated) onto a device substrate 704 (panel (a)). A layer ofprotective material 706 is subsequently deposited over the graphene(panel (b)). This protective material is desirably an organic material,such as uncrosslinked polystyrene, that prevents etchants used duringthe processing of the etch mask from reaching the underlying grapheneand that can be readily removed from the graphene after patterning. Ahard mask layer 708, such as an SiO₂ layer, is then deposited over theprotective layer (panel (c)). This hard mask layer serves as anetch-stop for etchants, such as an O₂ plasma, used to selectively etchdomains in the pattern-forming BCP layer and also prevents thecrosslinking of the underlying protective layer during the UV etching ofthe pattern-forming BCP layer. The pattern-forming BCP 710 is disposedon the hard mask layer (panel (d)). After the selective removal ofdomains in the pattern-forming BCP (panel (e)), the resulting patterncan be transferred through the hard mask layer using a suitable etchant(panel (f)). The pattern then can be transferred through the remainingprotective layer and the underlying graphene (panel (g)) using adifferent etchant system. Finally, the remaining portions of hard layerare removed (panel (h)), followed by the removal of the protective layer712 (panel (i)), leaving a nanoperforated graphene layer on a devicesubstrate.

EXAMPLES Example 1 Nanopatterning Graphene Using an Etch Mask with aSilicon Oxide Wetting Layer

In this example, a thin-film of the cylinder-forming diblock copolymerpoly(sytrene-block-methyl methacrylate)[P(S-b-MMA)] is used as atemplate to form nanoperforated graphene. In order to ensure the lateralphase segregation of the diblock copolymer into vertically orientatedcylinders on the graphene surfaces, two intermediate layers weredeposited on graphene. The first layer was a 10 nm silicon oxide filmwhich improved the wetting of the second layer, and the second layerconsisted of a thin film of a random copolymer of methyl methacrylate(MMA), styrene (S) and glycidyl methacrylate (GMA), P(S-r-MMA-r-GMA),(See, Han, E.; In, I.; Park, S. M.; La, Y. H.; Wang, Y.; Nealey, P. F.;Gopalan, P. Adv. Mater. 2007, 19, 4448.) which acted as anon-preferential or neutral layer. The intermediate random copolymerneutral layer adjusts the interfacial energies between the PS and PMMAblocks of the diblock copolymer and the underlying substrate leading tovertical orientation of the cylindrical domains for patterning. (See,Huang, E.; Pruzinsky, S.; Russell, T. P.; Mays, J.; Hawker, C. J.Macromolecules 1999, 32, 5299-5303.) As shown in FIG. 4, without thesilicon oxide film, the direct deposition of the P(S-r-MMA-r-GMA) onHOPG substrates resulted in non-uniform coating and hence subsequentdewetting of the overlying block copolymer film upon thermal annealing.

The P(S-r-MMA-r-GMA) random copolymer was synthesized bynitroxide-mediated living free radical polymerization as described inthe literature. (See, Han, E.; Stuen, K. O.; La, Y. H.; Nealey, P. F.;Gopalan, P. Macromolecules 2008, 41, 9090-9097.) The composition ofP(S-r-MMA-r-GMA) random copolymer was calculated by NMR analysis (S:70%, MMA: 26%, GMA: 4%). Number averaged molecular weight and PDI ofsynthesized P(S-r-MMA-r-GMA) were 46,700 g/mol, and 1.2, respectively.P(S-b-MMA) was used as the cylinder forming diblock copolymer(Mn_(PS)=46,000 g/mol, Mn_(PMMA)=21,000 g/mol, PDI=1.09) and waspurchased from Polymer Source, Inc. and used as received. HighlyOriented Pyrolytic Graphite (HOPG), Optigraph GmbH (Germany), was usedas a source for graphene. Initially, in proof-of-principle experiments,BCP lithography directly on the HOPG substrates was studied. Insubsequent experiments, mechanically exfoliated graphene monolayers werepatterned on SiO₂/Si substrates via BCP lithography to characterize theelectronic properties of the nanopatterned graphene.

To pattern HOPG, first, a 10 nm silicon oxide layer was deposited ontofreshly cleaved HOPG from a SiO₂ source (Telemark e-beam dielectricevaporator, Pressure: <2×10⁻⁶ torr, deposition rate: 1 Å/sec). Asolution of P(S-r-MMA-r-GMA) in toluene (0.3 wt %) was then spin-coatedon the silicon oxide buffer at 4,000 rpm and annealed at 160° C. undervacuum for 3 hours. The annealed sample was washed in toluene to removeuncrosslinked random copolymers, resulting in a 10 nm thick crosslinkedP(S-r-MMA-r-GMA) neutral layer.

Next, a block copolymer P(S-b-MMA) solution in toluene (1 wt %) wasspin-coated at 4,000 rpm onto the random copolymer-covered graphene andannealed (220° C., vacuum, 3 hours) resulting in a 25 nm thick film. Ahexagonal array of vertically oriented PMMA cylinders was observed atthis stage (FIG. 1 a). The sample was then exposed to UV illumination(1000 mJ/cm²) to selectively degrade the PMMA cylinders. PMMA residuewas removed by dipping in acetic acid for 30 minutes and rinsed with DIwater. Overall, this process yielded a hexagonally ordered nanoporous PStemplate (FIG. 1 b).

An O₂ plasma RIE (50 W, 10 mT, 10 sccm) was utilized to remove theunderlying P(S-r-MMA-r-GMA). The measured etch rates for PS andP(S-r-MMA-r-GMA) were 1.17 nm/sec and 1.42 nm/sec, respectively. CHF₃and O₂ mixed plasma RIE was then used to etch the oxide buffer layer.The etch rates of PS, P(S-r-MMA-r-GMA), and the oxide buffer were 0.76nm/sec, 0.96 nm/sec, and 1.09 nm/sec, respectively when a mixed gassystem (300 W, 60 mT, CHF₃ 45 sccm and O₂ 5 sccm) was used. FIG. 1 cshows that the honeycomb pattern, as well as the size of the hole (d)and the hole-to-hole period (D) were preserved between layers.Subsequently, the underlying HOPG was etched by O₂ plasma RIE (15 s). A10% HF aqueous solution was used to liftoff the oxide buffer layer. FIG.1 d shows the bare patterned HOPG following the removal of the polymersand oxide. The mode values of d, D, and graphite nanoribbon constrictionwidth (w) were 18.7 nm, 36.4 nm, and 17.8 nm, respectively as analyzedfrom the histogram using the ImageJ image analysis program (FIG. 2 a).

The nanoperforated HOPG was further characterized by x-ray photoelectronspectroscopy (XPS) and Raman spectroscopy. XPS spectra were collectedfrom 0 eV to 1000 eV using a Mg Ka X-ray source. Raman spectroscopy wasperformed with a 4 mW 633 nm HeNe laser source with <1 μm² spot size,and spectra were calibrated to the Si 520 cm⁻¹ peak. The XPS peaks Si2p(100 eV), Si2s (155 eV) and O1s (532 eV) observed before HF treatment(FIG. 2 bi), are attributed to the oxide buffer layer. After the removalof the oxide buffer, the Si peaks disappear, the intensity of O1s peakdecreases and the intensity of C1s peak (286 eV) increases (FIG. 2biii). The disappearance of the Si peaks confirms the complete removalof the oxide following the 10% HF aqueous treatment. The remaining O1speak suggests that the edges of the perforations in the HOPG are oxygenterminated as a result of the O₂ plasma etch or exposure to ambient. Toinvestigate the edge terminations during the intermediate, the XPSspectrum of patterned HOPG following the CHF₃+O₂ mixed plasma RIE wasmeasured. The final O₂ plasma etch step was eliminated but the oxide wasremoved with 10% HF aqueous solution. The presence of F1s (689 eV) peakand O1s (534 eV) peak confirms the termination of the hole edges are byF and O at this stage of processing (FIG. 2 bii).

Raman Spectroscopy was used as a non-destructive tool for probing theedges and the crystalline sp²-bonded structure of the HOPG afternanopatterning. (See, Ferrari, A. C. Solid State Comm. 2007, 143,47-57.) Prior to patterning, the G-(˜1580 cm⁻¹) and 2D-(−2600-2700 cm⁻¹)bands were prominent (FIG. 2 c). Generally, G-band scattering resultsfrom the degenerate E_(2g) phonon at the Brillouin zone center, which issensitive to doping in graphite, and the 2D-band is a double resonancefeature of the zone-boundary phonon around the K and K′ points, which isalso responsible for the singlet D-band. The D phonon is notRaman-active in HOPG and becomes active through double resonance orthrough a defect in the crystal such as a vacancy, edge, or grainboundary. Thus, the intensity of the D-band is a probe of disorder oredges.

After nanopatterning, the intensity of the D-band (linear combination ofLorentzians at 1318 and 1338 cm⁻¹) significantly increases (FIG. 2 c).Edge-disorder induced D′-band scattering (1613 cm⁻¹) is also detecteddue to intravalley double-resonance scattering. The D-band confirms thatthe BCP pattern has been transferred through the intermediate layers andthat the perforations have been etched into the HOPG resulting inubiquitous edges. The G-band and 2D-band show no shift compared with theunpatterned HOPG sample, indicating that the nanoperforated graphene,away from the edges, is not heavily doped and remains crystalline.

After characterizing the physical and chemical structure of thenanoperforated HOPG, the electronic properties of single sheets ofnanoperforated graphene monolayers were studied to investigate theeffects of quantum confinement and edge defects on their electronicstructure. Field effect transistor (FET) device geometries were utilizedto measure conductance and charge transport mobility as a function ofcarrier concentration.

To fabricate the devices, graphene was mechanically exfoliated onto 86nm SiO₂/Si (p++) wafers. (See, Blake, P.; Hill, E. W.; Neto, A. H. C.;Novoselov, K. S.; Jiang, D.; Yang, R.; Booth, T. J.; Geim, A. K. Appl.Phys. Lett. 2007, 91, 063124.) Monolayer pieces were identified byoptical phase contrast using predetermined correlations of opticalcontrast with measured spectral shifts of the Raman 2D bandcorresponding to mono-, bi-, and multiple layers of graphene. (See,Ferrari, A. C. Solid State Comm. 2007, 143, 47-57.) The degeneratelydoped Si substrate and the SiO₂ were used as the gate electrode anddielectric of the FETs, respectively. Four electrodes spaced at 1.2 μm(thicknesses: 4 nm Cr/26 nm Au/4 nm Ti) contacted the graphene. The topTi layer prevented sputtering of Au during subsequent etching.

After electrode patterning, 10 nm of silicon oxide was deposited on thegraphene (220° C. substrate, 50 μtorr O₂ backfill) to ensure copolymerwetting. To pattern the graphene monolayers, the final O₂ plasma etchwas shortened to 5 s and the HF etch was eliminated to prevent deviceliftoff. Low temperature measurements were performed using a He cryostatat 105 K at 2 E-4 torr, and room temperature measurements were performedin ambient. At room temperature, a four-wire geometry was utilized toeliminate the graphene/electrode contact resistances (FIG. 3 a).

The graphene-based devices (FIG. 3 b and FIG. 5) were electricallycharacterized at the various stages of the nanopatterning processing.Prior to patterning and to silicon oxide deposition, the devices showedexcellent charge transport characteristics and high mobility (>2,000 cm²V-1 s⁻¹) at room temperature (FIG. 3 c). The Cr/Au/Ti electrodesprovided ohmic contacts with contact resistances of 200-1000 ohms/μm².The graphene devices showed moderate hole-doping (as evidenced by theshift in the Dirac point) potentially due to extrinsic impurities suchas air molecule adsorption or residual PMMA. (See, Chen, J. H.; Jang,C.; Xiao, S. D.; Ishigami, M.; Fuhrer, M. S. Nat. Nanotechnol. 2008, 3,206-209.) The conductance ON/OFF modulation for gate biases in the rangeof ±30 V was ˜8 at room temperature. For gate potentials far from theDirac point, the mobility of all unpatterned devices (extracted fromσ=nqμ) was constant as a function of carrier concentration.

Following the deposition of the silicon oxide buffer layer, the chargetransport mobility significantly decreased to 220 cm2 V⁻¹ s⁻¹. TheON/OFF conductance ratio was 6.4. BCP deposition, UV etching, and PMMAremoval had no significant effect on the sample's electrical properties.The subsequent CHF₃ etching step, intended to etch through the oxidebuffer layer, decreased the absolute conductivity of the graphene (˜1/60) but increased the ON/OFF conductance ratio to ˜12 (FIG. 3 c).Following the CHF₃ etch, the Dirac point shifted >30V, indicatingsignificant hole-doping potentially as a result of edge fluorination(see F trace in FIG. 2C ii). Because of the large shift in the Diracpoint, the full ON/OFF ratio was not measured.

Subsequent O₂ plasma etching, which was intended to fully etch the holesin the graphene, shifted the Dirac point back towards neutralityallowing measurement of a larger ON/OFF conductance ratio of 41. Thesame device measured at T=105 K demonstrated an increased ON/OFF ratioof ˜207 with lower ON conductivity (FIG. 3 c).

The enhanced switching ratio of the nanoperforated graphene compared tothat of unpatterned graphene strongly suggests that an electronic bandand/or transport gap has been opened in the patterned material as aresult of quantum confinement, edge, and localization effects in theconstrictions. To characterize the effective gap of the nanoperforatedgraphene, the measured OFF conductance was related to an effective gapby assuming that the OFF conductance varies as the thermally-activatedcarrier concentration, which scales with temperature as ˜exp(−E_(G)/2k_(B)7), where E_(G) is the effective gap. The experimentally measuredOFF conductance follows an Arrhenius relationship with temperature,indicating that the opening of an effective gap of 102 meV as a resultof the nanopatterning (FIG. 3 c).

For comparison purposes, the nanoperforated graphene device with 18 nmconstrictions can be approximated as a mesoscopic honeycomb network ofnanoribbons with constriction widths ˜18 nm in which the ribbons areinterconnected between larger graphene islands. Tight-bindingcalculations of graphene nanoribbons predict multiple families ofelectronic band structure with significantly different band gapdependencies depending on the exact width and orientation. For example,nanoribbons of width 18 nm have predicted band gaps of 0-60 meV. (See,Castro Neto, A. H.; Guinea, F.; Peres, N. M. R.; Novoselov, K. S.; Geim,A. K. Rev. Mod. Phys. 2009, 81, 109-162.) More rigorous ab initiostudies that more closely match published experimental data on graphenenanoribbons have shown that all orientations of ribbons should have asignificant band gap. The ab initio calculations predict band gaps of80-250 meV for 18 nm ribbons, depending on orientation. (See, Yang, L.;Park, C. H.; Son, Y. W.; Cohen, M. L.; Louie, S. G. Phys. Rev. Lett.2007, 99, 186801.) Tight-binding calculations have also been developedto model the band gap of graphene anti-dot lattices similar to thepresent nanoperforated graphene membranes. The anti-dot latticecalculations predict a 67 meV band gap for the nanoperforated structure.(See, Pedersen, T. G.; Flindt, C.; Pedersen, J.; Mortensen, N. A.;Jauho, A. P.; Pedersen, K. Phys. Rev. Lett. 2008, 100, 136804.) Theeffective gap of 102 meV, measured here, qualitatively compares wellwith these calculations of the expected band gap.

Previous experimental studies have empirically correlated the observedswitching ratio and electronic band gap of graphene nanoribbonstransistors with the width of the nanoribbons. Han et al. have shownthat 24 nm ribbons have a switching ratio of 20 at T=100 K, suggesting aband gap of 27 meV. (See, Han, M. Y.; Ozyilmaz, B.; Zhang, Y. B.; Kim,P. Phys. Rev. Lett. 2007, 98, 206805.) Furthermore, Lin et al. havedemonstrated that 30 nm ribbons have a switching ratio of 9 at T=90 K,suggesting a band gap of 46 meV. (See, Lin, Y. M.; Perebeinos, V.; Chen,Z. H.; Avouris, P. Phys. Rev. B 2008, 78, 161409.) In comparison, themeasured effective gap of 102 meV exceeds these experimentally measuredband gaps for comparable constrictions.

In addition to the opening of an electronic band gap resulting fromquantum confinement effects, experimental and theoretical work has alsoshown that large conductance modulations in graphene nanoribbons canarise at low temperatures by several mechanisms other than an electronicband gap. Nominal edge roughness in carbon nanostructures has beentheorized to cause Coulomb blockade, strong localization leading toAnderson insulator-like behavior (see, Mucciolo, E. R.; Neto, A. H. C.;Lewenkopf, C. H. Phys. Rev. B 2009, 79, 075407), and the formation ofquasi-localized mid-gap states (see, Vanevic, M.; Stojanovic, V. M.;Kindermann, M. Phys. Rev. B 2009, 80, 045410) leading to a transport gap(see, Pereira, V. M.; dos Santos, J.; Castro, A. H. Phys. Rev. B 2008,77, 115109). Analysis of these effects is complex in the present systembecause of the large number of percolating current paths that exist inthe nanoperforated graphene membranes. The abrupt conductancemodulations (see, Sols, F.; Guinea, F.; Neto, A. H. C. Phys. Rev. Lett.2007, 99, 166803) near the Dirac point at T=105 K in FIG. 3 c mayindicate the formation of local resonances in the constrictions of thenanoperforated graphene giving rise to Coulomb blockade-like behavior,which has been experimentally demonstrated in graphene nanocontrictions.(See, Stampfer, C.; Gutttinger, J.; Hellmueller, S.; Molitor, F.;Ensslin, K.; Ihn, T. Phys. Rev. Lett. 2009, 102, 056403; Ponomarenko, L.A.; Schedin, F.; Katsnelson, M. I.; Yang, R.; Hill, E. W.; Novoselov, K.S.; Geim, A. K. Science 2008, 320, 356-358.)

It is believed that the observed conductance modulation is due mainly tothe opening of a sizable electronic band gap rather than Coulombblockade or localization effects. This hypothesis is supported by theobserved Arrhenius temperature dependence of the conductance, whichmatches the dependence expected for a traditional semiconductor with aband gap of 100 meV. Furthermore, this hypothesis is supported by theobservation of the enhanced switching ratio of the nanoperforatedgraphene at room temperature, and the agreement of the 100 meV extractedgap with the theoretical predictions of a quantum-confined band gap.Regardless of the mechanism, the effect of nanopatterning the grapheneis clear: an effective gap of 100 meV opens up, enabling conductancemodulation >40 and >200 at room temperature and at T=105 K,respectively.

Finally, to assess the relevance of nanoperforated graphene in potentialdevice applications, the hole mobility of the FETs was estimated using astandard transistor model (μ=gL/V_(DS)C_(ox)W), where g is thetransconductance, L and W are the channel length and width of theoriginal graphene sheet, respectively, V_(SD) is the source-drainpotential, and C_(ox) is the gate capacitance per unit area. Thecalculation describes the sheet mobility of the entire nanoperforatedgraphene layer and sets a lower bound to the mobility of the individualnanoconstrictions. After O₂ plasma etching, the device demonstrates ahole mobility of 1 cm² V⁻¹ s⁻¹ at low drain bias (10 mV) at roomtemperature. This value compares favorably with the free carriermobility of other thin film materials such as amorphous silicon, randompercolating nanotube networks, and organic semiconductors.

Example 2 Tailoring the Pattern Dimensions in Graphene by Altering theMolecular Weight of the Blocks in the Pattern-Forming BCP Layer

This example demonstrates how the dimensions of the pattern in a sheetof nanopatterned graphene can be reduced by reducing the molecularweights of the polymer blocks in a BCP pattern-forming layer.Specifically, although smaller constriction widths can be achieved withthe high molecular weight BCPs utilized in Example 1 by overetchingduring pattern transfer, a better approach to achieving smallerconstriction widths is to use BCPs with smaller molecular weights. BCPswith smaller molecular weights enable both smaller constrictions andsmaller periodicities, and therefore large materials fill factorresulting in potentially higher electrical current densities and highermechanical strength. Furthermore, better reliability and reproducibilitycan be achieved by avoiding overetching.

For this example, two additional P(S-r-MMA-r-GMA) random copolymers weresynthesized by nitroxide-mediated living free radical polymerization asdescribed in the literature. (Han, E.; Stuen, K. O.; La, Y. H.; Nealey,P. F.; Gopalan, P. 2008, 41, 9090-9097.) The composition ofP(S-r-MMA-r-GMA) random copolymers were calculated by NMR analysis. Theresulting compositions of the random copolymers were S: 82%, MMA: 14%,GMA: 4% (PG4-82) and S: 77%, MMA: 19%, GMA: 4% (PG4-77), respectively.Two kinds of P(S-b-MMA) block copolymers were used as the cylinderforming diblock copolymers (Mn_(Ps)=30,000 g/mol, Mn_(PMMA)=10,500g/mol, PDI=1.08, and Mnps=21,500 g/mol, Mn_(PMMA)=10,000 g/mol,PDI=1.06). These were purchased from Polymer Source, Inc. and used asreceived. Highly Oriented Pyrolytic Graphite (HOPG), Optigraph GmbH(Germany), was used as a source for graphene.

To pattern HOPG, first, a 10 nm silicon oxide layer was deposited ontofreshly cleaved HOPG from a SiO₂ source (Telemark e-beam dielectricevaporator, Pressure: <2×10⁻⁶ torr, deposition rate: 1 Å/sec). Asolution of PG4-82 in toluene (0.3 wt %) was then spin-coated on thesilicon oxide wetting layer at 4,000 rpm. In addition, a solution ofPG4-77 in toluene (0.3 wt %) was spin-coated on the silicon oxidewetting layer at 4,000 rpm. They were annealed at 160° C. under vacuumfor 3 hours. The annealed samples were washed in toluene to removeuncrosslinked random copolymers, resulting in a 10 nm thick crosslinkedPG4-82 and PG4-77 neutral layer.

Next, 30 k-10.5 k P(S-b-MMA) solution in toluene (0.8 wt %) wasspin-coated at 4,000 rpm onto the PG4-82 coated graphene and annealed(160° C., vacuum, 24 hours) resulting in a 22 nm thick film. 21.5 k-10 kP(S b-MMA) solution in toluene (0.8 wt %) was spin-coated at 4,000 rpmonto the PG4-77 covered graphene and annealed (160° C., vacuum, 24hours) resulting in a 20 nm thick film. The samples were then exposed toUV illumination (1000 mJ/cm²) to selectively degrade the PMMA cylinders.PMMA residue of those samples was removed by dipping in acetic acid for2 minutes and rinsed with DI water.

O₂ plasma RIE (50 W, 10 mT, 10 sccm) was utilized to remove theunderlying random copolymer layers. CHF₃ and O₂ mixed plasma RIE (300 W,60 mT, CHF₃ 45 sccm and O₂ 5 sccm) was then used to etch the oxidebuffer layer. Subsequently, the underlying HOPG was etched via an O₂plasma RIE. A 10% HF aqueous solution was used to liftoff the oxidebuffer layer.

The results of these studies are shown in FIG. 1, which shows SEM imagesof (a) vertically oriented polymethylmethacrylate (PMMA) cylinders inthe block copolymer thin films, (b) residual polystyrene (PS) honeycombtemplates obtained after selective PMMA removal with UV irradiation, (c)etched structures after O₂ followed by CHF₃+O₂ plasma reactive ion etch(RIE) resulting in the etching of the random copolymer mat and the oxidebuffer layer respectively, and (d) nanoperforated HOPG resulting fromthe final O₂ plasma RIE and the removal of the oxide wetting layer by HFsolution.

Example 3 Nanopatterning Graphene Using an Etch Mask with a SAM WettingLayer

This example demonstrates the use of a SAM as a wetting layer in agraphene etch mask used to fabricate a perforated graphene-based FET.

Single layer graphene was mechanically exfoliated onto 86 nm SiO₂/Si(p++) wafers. Monolayer pieces were identified by optical phase contrastusing predetermined correlations of optical contrast with measuredspectral shifts of the Raman G′ band corresponding to mono-, bi-, andmultiple layers of graphene. The degenerately doped Si substrate and theSiO₂ were used as the gate electrode and dielectric of the FETs,respectively. Four electrodes spaced at 1˜1.2 μm (thicknesses: 4 nmCr/26 nm Au/4 nm Cr) were defined by e-beam lithography and thermalmetal evaporation to contact the graphene.

1-Pyrene butyric acid (97%) was purchased from Aldrich and used asreceived. To form a self-assembled monolayer of 1-pyrene butyric acid onthe graphene, graphene FET devices were immersed in 10 mM solution of1-pyrene butyric acid in ethyl alcohol for 24 hrs. The treated deviceswere washed with ethyl alcohol.

Then, 0.3 wt % toluene solutions of P(S-r-MMA-r-GMA) random copolymers,(PG4-70, PG4-77 and PG4-82) were spin-coated on 1-pyrene butyricacid/graphene devices and annealed at 160 V under vacuum for 3 hrs. Theannealed samples were washed in toluene to remove uncrosslinked randomcopolymers, resulting in a 10 nm thick crosslinked PG4-70, PG4-82 andPG4-77 neutral layer on graphene.

46 k-21 k P(S-b-MMA) solution in toluene (1 wt %), 30 k-10.5 kP(S-b-MMA) solution in toluene (0.8 wt %), and 21.5 k-10 k P(S b-MMA)solution in toluene (0.8 wt %) were spin-coated at 4,000 rpm onto thePG4-70, PG4-82, and PG4-77 coated graphene devices, respectively. 46k-21 k block copolymer sample was annealed at 220 V under vacuum for 2hrs. The samples of 30 k-10.5 k and 21.5 k-10 k P(S-b-MMA) were annealedat 160 V under vacuum for 24 hrs Annealed samples were then exposed toUV illumination (1000 mJ/cm²) to selectively degrade the PMMA cylinders.PMMA residue of those samples was removed by dipping in acetic acid for2 minutes and rinsed with DI water.

O₂ plasma RIE (50 W, 10 mT, 10 sccm) was utilized to remove theunderlying random copolymer layers and to etch through graphene tofabricate a nanoperforated structure.

The results of these studies are presented in FIG. 8, which shows SEMimages of the resulting etch masks on graphene (top panels) and thenanoperforated graphene after the removal of the etch mask (bottompanels).

Example 4 Solvent Annealing

This example demonstrates the use of solvent annealing to form an etchmask for nanopatterning graphene.

Single layer graphene was mechanically exfoliated onto 86 nm SiO₂/Si(p++) wafers. Monolayer pieces were identified by optical phase contrastusing predetermined correlations of optical contrast with measuredspectral shifts of the Raman G′ band corresponding to mono-, bi-, andmultiple layers of graphene. The degenerately doped Si substrate and theSiO₂ were used as the gate electrode and dielectric of the FETs,respectively. Four electrodes spaced at 0.3˜0.35 μm (thicknesses: 4 nmCr/26 nm Au/4 nm Cr) were defined by e-beam lithography to contact thegraphene.

PMMA cylinder forming P(S-b-MMA) block copolymer was synthesized byliving anionic polymerization. Number average molecular weights of PSblock and PMMA block of the copolymer were 15,000 g/mol and 10,000g/mol, respectively. A solution of P(S-b-MMA) block copolymer in toluene(0.8 wt %) was spin-coated at 4,000 rpm on mechanically exfoliatedgraphene FET device. The film was placed in a CS₂-saturated annealingchamber at room temperature for 10 hrs to expose block copolymer thinfilm to CS₂ vapor. During the exposure to CS₂ vapor, perpendicular PMMAcylindrical structure was formed. The sample was then exposed to UVillumination (1000 mJ/cm²) to selectively degrade the PMMA cylinders.PMMA residue of those samples was removed by dipping in acetic acid for1 minutes and rinsed with DI water. An O₂ plasma RIE (50 W, 10 mT, 10sccm) was utilized to etch through graphene to fabricate nanoperforatedstructure.

Example 5 Three-Layered Approach for Large Area Nanopatterning

This example demonstrates the “tri-layer” approach for nanopatterninggraphene.

Large Area Graphene Synthesis and Transfer to SiO₂/Si Substrate:

Large-area monolayer graphene was grown in a horizontal CVD furnace witha 32 mm ID quartz tube. Copper foil (Alfa Aesar, product #13382) wasused as the growth catalyst. The foil was heated to 1050° C. under theflow of 900 sccm forming gas (95% argon, 5% hydrogen) and annealed underthe same conditions for 30 minutes. The furnace was then cooled to 1020°C., whereupon methane was introduced at 10 ppm and graphene was allowedto grow for 16 hours. The foil was then rapidly cooled at ˜10° C./sec to700° C. and then allowed to cool to room-temperature.

Graphene was transferred to an 86 nm SiO₂/Si (p++) wafer by firstspinning a solution of PMMA (Microchem, product 950 PMMA 2C) onto thecopper foil. Then, the foil was floated on an aqueous solution of 0.2MHC1, 0.2M FeC13 for 10 hours to dissolve the copper, leaving the PMMAand graphene film floating on the solution. The film was washed withde-ionized water (DI water) and transferred to a solution of 1:9 HF(49%): DI water, and allowed to etch for 40 minutes. The film was againwashed with DI water and allowed to dry on a 86 nm SiO₂/Si (p++) wafer.The PMMA film was removed using boiling dichloromethane, and the waferwas then washed with iso-propanol and blown dry with nitrogen, leaving amonolayer graphene film on the SiO₂/Si wafer.

Tri-Layer BCP Pattern on Large-Area Graphene:

A tri-layer strategy was used to pattern the large-area graphene. In thetri-layer strategy, the graphene was first covered by a layer ofpolystyrene—which is referred to as the protective layer. Thepolystyrene layer was then covered by a SiO₂ layer, which we call thehard-mask layer. A BCP layer was formed on a dummy substrate (describedbelow) and then floated onto the graphene/polystyrene/SiO₂.

The polystyrene protective layer and the SiO2 hard mask layer served 3purposes:

(1) Protection. The polystyrene layer acts as protective layer toprevent the direct etching of the graphene by the fluorine gas plasma.

(2) Hole-enlargement: By first patterning holes in the SiO₂ hard-mask,it is then possible to controllably enlarge the hole size in theunderlying polystyrene protective layer.

(3) Facilitation of post-patterning polystyrene removal: In the absenceof the protective layer, the polystyrene layer left over from theoriginal BCP layer is highly cross-linked due to its exposure to the O₂plasma etchant. The cross-linked nature of this polystyrene inhibits itsremoval which is undesirable because the nanoperforated graphene is thenovercoated by an irremovable and insulating polystyrene layer. Incontrast, in the tri-layer strategy, the SiO₂ hard-mask layer shieldsthe top-surface of the polysytrene from the O₂ plasma etchant,preventing its cross-linking. Therefore, it can be removed in solventafter pattern transfer into the underlying graphene.

Details of Layer Deposition and Pattern Transfer:

To form the “protective layer” a solution of polystyrene (Mn=20,000g/mol) in propylene glycol methyl ethyl acetate was spin-coated at 4,000rpm onto CVD grown single layer graphene on 86 nm SiO₂/Si (p++) wafersand annealed at 140° C. for 5 min to remove residual solvent. A 10 nmsilicon oxide layer was deposited onto polystyrene coated graphenesample from a SiO₂ source by e-beam evaporation. (Telemark e-beamdielectric evaporator, Pressure: <2×10⁻⁶ torr, deposition rate: 0.5Å/sec)

To deposit a block copolymer film on the silicon oxide/PS/graphenesubstrate, a floating and transferring technique was used in which a BCPfilm was formed on a separate “dummy” substrate and then transferred tothe silicon oxide/PS/graphene substrate. First, PE-CVD (Plasma Therm 74)was performed in order to deposit 150 nm of silicon oxide on a “dummy”silicon wafer. Then, a 1 wt % solution of hydroxyl terminated P(S-r-MMA)random copolymer (S: 70%, MMA: 30%) in toluene was spin-coated at 1,000rpm, and annealed at 220° C. for 6 hrs under vacuum. Hydroxyl terminatedP(S-r-MMA) random copolymer was synthesized as reported earlier. (P.Mansky, Y. Liu, E. Huang, T. P. Russell, C. Hawker, Science 1997, 275,1458.) The annealed sample was washed with toluene to remove unreactedrandom copolymers. A block copolymer, 46 k-21 k, P(S-b-MMA) solution intoluene (1.5 wt %) was spin-coated at 4,000 rpm onto the randomcopolymer-covered silicon oxide/silicon wafer and annealed at 230° C.under vacuum for 3 hrs, resulting in perpendicular PMMA cylindricalstructure of block copolymer thin film.

In order to float the block copolymer film on an air-water interface, a20% HF aqueous solution was used to remove the random copolymer layerand the SiO₂ layer. The floated block copolymer film on air-HF aqueoussolution interface was then transferred to DI water. Then, the floatedblock copolymer film at the air-water interface was picked up with asilicon oxide/PS/graphene substrate, resulting in block copolymer,silicon oxide, and PS trilayer structure on graphene.

The sample was then exposed to UV illumination (1000 mJ/cm²) toselectively degrade the PMMA cylinders. PMMA residue of those sampleswas removed by dipping in acetic acid for 2 minutes and rinsing with DIwater.

O₂ plasma RIE (50 W, 10 mT, 10 sccm) was utilized to remove residuesinside the holes and the underlying random copolymer layer. CHF₃ and O₂mixed plasma RIE (300 W, 60 mT, CHF₃ 45 sccm and O₂ 5 sccm) was thenused to etch the oxide buffer layer. Subsequently, the underlyingpolystyrene layer and graphene were patterned via O₂ plasma RIE (20 W,15 mT, 8 sccm) with the patterned silicon oxide hard mask. A 1% HFaqueous solution was used to liftoff silicon oxide on polystyrene layer.Finally, NMP, acetone and isopropyl alcohol were used to remove etchedpolystyrene residue.

Deposition of Metal Electrodes on Top of the Nanoperforated Graphene:

After the BCP removal, gold electrodes were deposited directly on thenanoperforated graphene through a shadow mask. The nanoperforatedgraphene was then patterned into arrays of channels that were ˜120 μmwide and ˜12 μm long. To achieve this, a sacrificial copper etch maskwas deposited through a shadow mask such that there was a copper layereverywhere that the nanoperforated graphene was to be preserved. Then,the exposed nanoperforated graphene that was not covered by either thegold or copper layers was removed using a 10 sec O₂ plasma RIE (50 W, 10mT, 10 sccm), and subsequently the copper was removed using an aqueoussolution of 0.2M HC1, 0.2M FeC13 for one minute. The nanoperforatedgraphene was then cleaned using NMP and iso-propanol. This procedureresulted in channels of nanoperforated graphene ˜120 μm width and ˜12 μmlength that were contacted at both ends of the channel by goldelectrodes.

As used herein, and unless otherwise specified, “a” or “an” means “oneor more.” All patents, applications, references, and publications citedherein are incorporated by reference in their entirety to the sameextent as if they were individually incorporated by reference.

As will be understood by one skilled in the art, for any and allpurposes, particularly in terms of providing a written description, allranges disclosed herein also encompass any and all possible subrangesand combinations of subranges thereof. Any listed range can be easilyrecognized as sufficiently describing and enabling the same range beingbroken down into at least equal halves, thirds, quarters, fifths,tenths, etc. As a non-limiting example, each range discussed herein canbe readily broken down into a lower third, middle third and upper third,etc. As will also be understood by one skilled in the art, all languagesuch as “up to,” “at least,” “greater than,” “less than,” and the likeincludes the number recited and refers to ranges which can besubsequently broken down into subranges as discussed above. Finally, aswill be understood by one skilled in the art, a range includes eachindividual member.

It is specifically intended that the present invention not be limited tothe embodiments and illustrations contained herein, but include modifiedforms of those embodiments including portions of the embodiments andcombinations of elements of different embodiments as come within thescope of the following claims.

1. An etch mask structure for a graphene material, the structurecomprising an etch mask disposed over a graphene material, wherein theetch mask comprises a wetting layer in contact with the graphenematerial, a neutral layer comprising a copolymer disposed over thewetting layer, and a pattern-defining block copolymer layer disposedover the neutral layer, wherein the pattern-defining block copolymer isphase segregated into a pattern of domains; and further wherein theneutral layer is characterized in that is induces the formation of thepattern of domains in the pattern-defining block copolymer.
 2. Thestructure of claim 1, wherein the wetting layer is a layer of siliconoxide.
 3. The structure of claim 2, wherein the layer of silicon oxidehas a thickness of about 5 to about 20 nm.
 4. The structure of claim 2,wherein the neutral layer comprises a copolymer polymerized from vinylmonomers and acrylate monomers.
 5. The structure of claim 4, wherein thepattern-defining block copolymer is polymerized from vinyl monomers andacrylate monomers.
 6. The structure of claim 5, wherein thepattern-defining block copolymer is P(S-b-MMA).
 7. The structure ofclaim 1, wherein the wetting layer comprises a self-assembled monolayer.8. The structure of claim 7, wherein the self-assembled monolayercomprises molecules having a first end that is sufficiently polar toundergo an attractive interaction with the copolymer of the neutrallayer and a second end comprising a plurality of conjugated rings thatundergo a π-π stacking interaction with the graphene material.
 9. Thestructure of claim 8, wherein the first end of the molecules comprise anacid group and second end of the molecules comprise at least three fusedsix-member aromatic rings.
 10. The structure of claim 9, wherein themolecules are pyrene butyric acid molecules.
 11. The structure of claim7, wherein the neutral layer comprises a copolymer polymerized fromvinyl monomers and acrylate monomers.
 12. The structure of claim 11,wherein the pattern-defining block copolymer is polymerized from vinylmonomers and acrylate monomers.
 13. The structure of claim 12, whereinthe pattern-defining block copolymer is P(S-b-MMA).
 14. The structure ofclaim 1, wherein the graphene material consists of a single layer ofgraphene.
 15. The structure of claim 1, wherein the graphene materialcomprises highly oriented pyrolytic graphite.
 16. The structure of claim2, wherein the graphene material consists of a single layer of graphene.17. The structure of claim 2, wherein the graphene material compriseshighly oriented pyrolytic graphite.
 18. The structure of claim 7,wherein the graphene material consists of a single layer of graphene.19. The structure of claim 7, wherein the graphene material compriseshighly oriented pyrolytic graphite.